1. Field of the Invention
The present invention relates to a heat-generator supporting member for a recording head of an ink-jet recording system, which is excellent in durability, suitable for mass production, and particularly suitable for a long head such as a full-line type head.
2. Related Background Art
The ink-jet system is attracting attention in recent years owing to capability for printing at a high speed and a high density, suitability for color printing, and compactness of the system.
FIG. 1 shows the construction of an ink-discharging portion and its vicinity of a typical conventional ink-jet head.
In FIG. 1, the numeral 1 denotes a support which supports structurally a heat-generating resistor. The support is required to have a high thermal conductivity for diffusing excess heat generated at the heat-generating resistor to rapidly cool a heat-generating portion and its vicinity to a certain temperature or lower after completion of the pulse application for bubbling and discharge. The smooth surface of the support and the smooth surface of the heat-generating portion are important for providing uniform bubbling with high energy at the heat-generating portion for a stable ink discharge. Furthermore, the flat surface of the support is an important factor for forming a fine pattern for heat-generating resistors, electrodes, nozzles, and the like. As the support, a single-crystal silicon wafer is usually used.
In FIG. 1, the numeral 2 denotes a heat-accumulating layer. The heat-accumulating layer inhibits heat transfer to the support and allows the generated heat to efficiently act on the ink. Therefore, the heat-accumulating layer requires a low thermal conductivity in contrast to the support. The heat-accumulating layer requires heat resistance since it is in direct contact with the heat-generating resistor. Furthermore, the heat-accumulating layer requires an insulating property to inhibit electric conduction between the resistors and between electrodes. The heat-accumulating layer is usually formed from an insulating material, in particular, silicon oxide having a low heat conductivity, or the like.
In FIG. 1, the numeral 3 denotes a heat-generating resistance layer (heat-generating resistor). The heat for bubbling of the ink is generated by application of electric pulses to the portion of the heat-generating resistance layer 3 between a common electrode 4 and individual electrodes 4' each connected electrically to a driving circuit and a power source (not shown in the drawing). The heat-generating resistor requires heat resistance, suitable specific resistivity, and stability of the resistance, and is usually made of materials such as HfB.sub.2, TaAl, or the like. The electrodes 4, 4' are usually made of a material having a low resistivity such as Al on Au.
In FIG. 1, the numeral 5 denotes a protective layer. The protective layer separates the electrodes and the heat-generating resistor from an electroconductive ink to carry out insulation from the ink and to prevent electrochemical damage from being caused by the ink. The protective layer serves also as an oxidation inhibiting layer for the heat-generating resistor. The protective layer requires heat-resistant and insulating properties, and is usually made of materials such as silicon oxide, silicon nitride, or the like.
In FIG. 1, the numeral 6 denotes an anti-cavitation layer. The anti-cavitation layer protects the heat-generating resistor from damage caused by cavitation upon extinction of the bubbles. The anti-cavitation layer is usually made of a metal having a strong resistance to cavitation erosion such as Ta. The anti-cavitation layer also requires chemical stability in addition to the anti-cavitation-erosion property since it is brought into contact with the ink at a high temperature.
Additionally, a protective layer (e.g., organic protection layer, etc.) may be optionally provided by coating on a region except the heat-generating portion to prevent damage to the electrode caused by pinholes in the protective layer 5 and the anti-cavitation layer 6.
On such a heat-generator supporting member 7, liquid flow paths 8 each corresponding to respective heat-generating resistors and communicating to a recording liquid feed opening (not shown in the drawing), and discharging orifices 9 are formed to complete an ink-jet head 10.
The constitution of the heat-generator supporting member at and around the discharging portion is similar to that of a conventional thermal head. However, the thermal and chemical characteristics required for the heat-generator supporting member are severer than the characteristics required for the thermal head because it is in direct contact with a liquid, because it is subjected to mechanical impact (cavitation erosion) caused by repeated bubble formation and bubble extinction, and because it is subjected to increase and decrease of temperature of several 100 .degree. C. to 1000 .degree. C. in a short time of several microseconds.
By a fine pattern formation technique such as photolithography, the ink-jet head having the aforementioned constitution can readily be formed with higher density of the head (several tens of nozzles per millimeter) and higher integration degree (several hundred nozzles per head). Therefore, it can perform recording of very high quality and higher speed recording owing to the especially higher frequency (several to several-ten kHz) of discharging ink droplets in comparison with a recording speed of thermal head or the like. The aforementioned ink-jet head has many excellent characteristics such as applicability to color recording, compactness of the head, and low running cost.
It is required that the ink-jet head having excellent characteristics as mentioned above has capability of higher speed recording as the results of the progress in processing ability of computers and the increase of the amount of information.
For higher speed recording, the number of the nozzles of the head may be increased simply. However, the increase of the number of the nozzles results in increase of the length of the head, causing the problems discussed below.
Conventionally, a single-crystal Si wafer is used as the support, as mentioned above. The single-crystal Si wafer has advantages that it is readily available, and yet has high heat resistance, high thermal conductivity, high surface smoothness, and high planarity. It can be processed by a film-forming apparatus, a patterning apparatus, and the like of a conventional semiconductor process. The driving IC can be incorporated into the support, and so forth. However, a long one-chip head is not producible from the single-crystal Si wafer, disadvantageously.
From an 8-inch Si wafer readily available at the moment, a full-line type head of A4-size paper sheet breadth cannot be produced as one chip. A larger wafer is required therefor. However, general type processing machines cannot process such a larger wafer. A less number of such long supports can be produced from a disk-shaped wafer, which results in a remarkable increase of the cost. Therefore, to produce a long head from an Si wafer, it is necessary to construct one long head from a plurality of chips (supports), or one long head from a plurality of heads. However, the positional registration cannot readily be conducted. The difficulty in the registration increases greatly with increase of the recording density and to meet the requirement for higher recording quality. Therefore, a material for a long support in place of the Si wafer is demanded for production of a long ink-jet head.
It is required that a heat-accumulating layer 2 composed of a low thermal conductivity is provided in a layer as shown FIG. 1 as mentioned above. When an Si wafer is employed as the support, a thermally oxidized SiO.sub.2 film formed by modifying the surface of the wafer itself can be used as the heat-accumulating layer. Otherwise, when the support of a metal or the like is employed, it is necessary to form on the support a film of a material such as silicon oxide or the like having an insulating property, a heat resistant property, and a low thermal conductivity by sputtering, CVD, or a like method since the modified surface of the support has usually high thermal conductivity unsuitable for the heat-accumulating layer. However, the film formed by physical vapor deposition, chemical vapor deposition, coating-and-firing, or the like is generally inferior in film properties such as chemical stability in comparison with thermally oxidized SiO.sub.2.
It was found by the inventor of the present invention that the difference of the film quality can greatly affect the durability of the heat-generating resistor of the ink-jet head. A lower film quality of the heat-accumulating layer tends to increase the change of resistivity of the heat-generating resistor of the head during the driving, resulting in early breakdown. It is necessary to form the heat-accumulating layer having a film quality as high as possible.
When the heat-accumulating layer of the ink-jet head for effective discharge is formed from silicon oxide or the like, it requires at least a thickness ranging from 1.5 .mu.m to 2 .mu.m depending on the driving pulse breadth. With such a dimension of the thickness, the stress built up in the film at the film formation affects greatly the support. For example, excessive stress in the film may cause warpage of the support, or exfoliation of the film to cause a serious problem in the production process.
Accordingly, it is necessary to control the film stress in the heat-accumulating layer as well as the film quality. The control of the film stress is more important for a longer head and a larger support. Generally, the quality of the film formed by sputtering, CVD, or the like can be improved by elevating the support temperature during the film formation, or by formation of the film with application of impact with ions. However, the film formation at a higher support temperature causes stronger internal stress at room temperature owing to the difference between the thermal expansion coefficient of the support and the film material to cause warpage of the support. Thus, simultaneous control of the high film quality and the weak film stress is often achieved with large difficulty.
A conventional ink-jet head has also a protective film made of silicon oxide, silicon nitride, or the like on the heat-generating resistor and the electrode as shown in FIG. 1. In such a type of ink-jet head, it is extremely difficult to prevent completely the generation of defects (pinholes) caused during the protection film formation, which lowers the yield of the ink-jet heads in mass production. This problem becomes more serious with the increase of the length and area of the head. The formation of pinholes can be prevented to some extent by increasing the thickness of the protective layer. However, the larger thickness of the protective layer leads to a larger power consumption for ink discharge and a larger temperature change of the entire head on driving. The temperature change of the head causes a change of the volume of the discharged liquid, resulting in image density irregularity.
Increase of the driving frequency for increasing the recording speed causes further increase of the power consumption of the head to make more remarkable the irregularity of the image density by temperature variation. This is contradictory to the requirement for higher quality of the recorded image and is a problem to be solved.
Japanese Patent Application Laid-Open No. 59-143650 and No. 60-109850 disclose that the surface layers of the heat-generating resistor and the electrodes are modified into an inorganic insulating layer to use it as the protective layer. According to this method, it is possible to form a thin protective film without defects. This method is very excellent in easy treatment of a larger area of the layer and simple process.
The inventor of the present invention found formerly that the above method is more effectively carried out by using the heat-generating resistor of a Ta-Al alloy.
Further, the inventor of the present invention found that the aforementioned problem of the film quality of the heat-accumulating layer is more serious in durability of the heat-generator supporting member for an ink-jet head produced by the above method. That is, the quality of the heat-accumulating layer is directly reflected to the durability of the heat-generating resistor in the heat-generator supporting member for the ink-jet head having a thin protective film formed by modification of the heat-generating resistor or electrode itself in place of the conventional thick protective film made of silicon oxide, silicon nitride, or the like. Therefore, the requirement for the accumulating layer quality is more severe in this type of the heat-generator supporting member. This is a great obstacle in a long head in which a thermally oxidized SiO.sub.2 film of high quality cannot be utilized as the heat-accumulating layer.